The invention relates to a capped high-pressure discharge lamp comprising:
a light-transmitting lamp vessel which is closed in a vacuumtight manner and which encloses a discharge space with an ionizable filling, wherein electrodes are arranged in mutual opposition so as to define a discharge path between them, which lamp vessel has an axis and a first and a second neck-shaped portion with a seal;
a glass tubular outer envelope around the discharge space, surrounding the discharge vessel with clearance, which envelope is connected to the lamp vessel and is filled with gas;
a lamp cap which is provided with contacts and in which the first neck-shaped portion is secured;
a first and a second current conductor, each connected to a respective electrode and extending through the respective first and second neck-shaped potion respectively to a respective contact at the lamp cap, the second current conductor having a return portion which extends along an outside of the outer envelope.
Such a capped high-pressure discharge lamp is known from European Patent Application EP-0 570 068-A1.
The outer envelope of the lamp is useful for reducing the temperature differences of the lamp vessel during operation. Reduced temperature differences can raise the luminous flux of the lamp while the power consumption remains the same. A rise in the luminous flux can even be realised at a maximum lamp vessel temperature corresponding to the maximum temperature in the absence of the envelope. It is also possible to realise a reduction in the maximum temperature while maintaining the same luminous flux as that given by a non-enveloped lamp. The luminous flux and the maximum temperature are dependent on the value of the clearance between the outer envelope and the lamp vessel.
The known lamp is particularly suitable for use as a vehicle headlight lamp. The light source of the lamp, the discharge are, has a high brightness while the consumed power of approximately 35 W is converted into light with a comparatively high luminous efficacy. As a result, the lamp can be used in a headlight with a comparatively small reflecting surface, so that the headlight can have a comparatively small height. Nevertheless, a more brightly illuminated road surface is then obtained for the vehicle with the lamp inside than is the case with conventional incandescent lamps as the light source. It was found, however, that a headlight with the capped high-pressure discharge lamp may radiate stray light, which may give rise to glare, also owing to the high brightness of the light source and the high luminous flux, and also dependent on the type of headlight.
Capped high-pressure discharge lamps of the kind described are also known from EP-0 581 354-A1, EP-0 579 326-A1, and EP-0 579 313-A1.
EP-0 237 647-A1 discloses a high-pressure discharge lamp which can be used as a vehicle headlight lamp and whose lamp vessel is surrounded by an evacuated tubular outer bulb of hard glass. This bulb is provided with a radiation-absorbing coating over its entire length and over half its circumference minus approximately 15.degree., and is provided with an IR-reflecting, light-transmitting coating over its remaining surface area.
The radiation-absorbing coating over a circumferential angle of 180.degree.-15.degree. has for its object to create an asymmetrical light beam in a headlight with a paraboloidal reflector for right-hand and left-hand traffic.
It was found that such a coating in a lamp of the kind mentioned in the opening paragraph, even with the use of quartz glass for the outer envelope and even in the absence of an IR-reflecting coating, causes such high temperatures that lamp life is strongly reduced, the tureen output of the lamp drops quickly, and deformations occur.